1. Field of the Invention
The present invention relates to a multiplex communication method for implementing communication, e.g., between a plurality of devices mounted on a vehicle as well as to a multiplex communication apparatus to which the multiplex communication method is applied.
2. Description of the Related Art
A conventional method and apparatus will be described taking a passenger protector shown in FIG. 3 as an example. This example is characterized as including a front passenger protector (first circuit unit) 14 that has the function of protecting passengers from a frontal crash and a side passenger protector (second circuit unit) 15 that has the function of protecting passengers from a sideward collision.
In FIG. 3, a booster circuit 3 boosts an input voltage from a battery 1 supplied through an ignition switch 2 and charges a back-up capacitor 5 through a resistor 4. The electric charges stored in the back-up capacitor 5 is discharged to a detonator 8 and a mechanical acceleration switch 9 (turned on at this time) in series through a discharge diode 6 by a microcomputer 11 turning a switch circuit 7 on upon judgment that a grave collision has occurred from an acceleration signal applied from a lengthwise acceleration sensor 10. As a result, a not shown explosive is ignited by the detonator 8 and air bags are expanded. It may be noted that the mechanical acceleration switch 9 is of such a structure as disclosed in Japanese Patent Application No. Hei. 5-351470 by the present applicant.
Further, the microcomputer 11 has a troubleshooting function. When making a capacitance diagnosis on the back-up capacitor 5, the microcomputer 11 turns a transistor 13 on to discharge the electric charges stored in the back-up capacitor 5 through a resistor 12, and reads an amount of change (per unit time) of the discharge voltage of the back-up capacitor 5 at this time. This is how the microcomputer 11 performs the capacitance diagnosing operation. When the microcomputer 11 judges that there exists an abnormality, the microcomputer 11 informs passengers of the abnormality using a not shown alarm unit such as a lamp.
The side passenger protector 15 will be described next.
That is, in FIG. 3, the side passenger protector (the one on the right out of those shown by the broken lines in FIG. 3, and this protector is also called a side air bag) 15 has a power supply fed thereto while connected to the back-up capacitor 5 of the main passenger protector 14 by a power supply line such as a wiring harness shown by reference numeral 16.
The side passenger protector 15 is mounted on or close to a door of a vehicle. Reference numeral 10' denotes a widthwise acceleration sensor, which is the same as the lengthwise acceleration sensor 10. The widthwise acceleration sensor 10' is mounted on the vehicle in a different direction so that acceleration applied across the width of the vehicle can be detected.
Reference numeral 11' denotes a microcomputer that has a collision judging function equivalent to that of the microcomputer 11. When the microcomputer 11' judges that a grave collision has occurred based on an acceleration signal outputted from a widthwise acceleration sensor 10' derived from a sideward collision, the microcomputer 11' controls a switch circuit 7' to be turned on. It may be noted that reference numeral 8' denotes a detonator equivalent to the detonator 8; 9', a mechanical acceleration switch equivalent to the mechanical acceleration switch 9; 17, a constant voltage circuit. The constant voltage circuit 17 is designed to apply a predetermined voltage to various circuits constituting the side passenger protector 15 in response to an output from the booster circuit 3 (or the back-up capacitor 5) through the power supply line 16.
That is, the microcomputer 11 of the front passenger protector 14 controls the transistor 13 to be kept turned on for a predetermined period of time when the ignition switch 2 has been turned on, and diagnoses the capacitance of the back-up capacitor 5. When the capacitance of the back-up capacitor 5 is judged to be abnormal, the microcomputer 11 causes the not shown alarm unit to issue an alarm, whereas when the capacitance of the back-up capacitor 5 is judged to be normal, the microcomputer 11 receives a detected output from the front side acceleration sensor 10 and judges whether or not a collision has occurred. If it is judged that a grave collision has occurred, the microcomputer 11 controls the switch circuit 7 to be kept turned on for a predetermined period of time, and discharges the electric charges stored in the back-up capacitor 5 to the detonator 8 to thereby expand the air bags and the like.
Further, the side passenger protector 15 not only receives a boosted voltage from the booster circuit 3 of the front passenger protector 14 through the power supply line 16, but also causes the widthwise acceleration sensor 10' to detect acceleration attributable to a sideward collision. When the microcomputer 11' judges that a grave accident has occurred from the detected signal from the sensor 10', the microcomputer 11' controls the switch circuit 7' to be kept turned on, causes power stored in the back-up capacitor 5 of the front passenger protector 14 to flow to the detonator 8' and the acceleration switch 9' in series, and ignites the explosive to thereby expand the air bags. It may be noted that both switches 7, 7' are never turned on simultaneously in ordinary cases.
Thus, in the conventional example, both the front passenger protector 14 and the side passenger protector 15 share only the power supply in common and process signals at separate circuits; i.e., their signals are not intercommunicated, which has imposed the problem of high cost.